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Why doesn't the A330/A340 have a stickshaker?
When an A330/40 is in alternate law1 and approaching a stall, it has a stall warning composed entirely of a cricket chirp and a voice saying "stall, stall".  Most other aircraft, in contrast, have a stickshaker, which vibrates the stick when the aircraft approaches a stall.
Pilots are, as demonstrated by numerous crashes, very (very very) good at tuning out aural warnings, whereas a stickshaker provides a powerful tactile cue that is very difficult to ignore; so why doesn't the A330/40 have a stickshaker?
1: In normal law, the A330/40 has no stall warning, as its automatic angle-of-attack protections supposedly make it impossible to stall the aircraft by pulling on the stick.

Stall warning operates independently from the high-AOA protection and does sound in normal law. This would happen in highly dynamic situation where alpha/speed variations are large, especially when operating close to maximum weight.

Without any knowledge about certification of the sidestick controller, I would speculate that shaking the controller would hamper the controllability and disturb the movements on the sidestick. The range of motion on the sidestick is much smaller than on conventional yoke and required deflections are often so small that shaker would actually prevent accurate flying in the critical moment.

On Airbus aural warnings can’t be switched off

@user40476: I said tuning out, not turning off.

Sorry,  thank you, but you can’t ignore the STALL aural alarm it’s so loudly. Further the stick displacement is very narrow compared to a yoke, a stick shaker will produce oscillating inputs to the stick transducers.

@user40476: You'd be surprised at what pilots can ignore.

Having flown the 737 (with a stick shaker) and A320 (without a stick shaker) for several years, two things come to mind:

The aural warning is VERY loud and distinct. I can hardly imagine that anyone would miss it.
In your type training, and sometimes in your recurrent trainings, you practice stall recoveries, and this sound seems to be stuck and hard-wired to your brain after a few times.
If the sidestick would vibrate, it would be very hard to control the aircraft properly. The stick is very sensitive to even small deflections, whereas in conventional aircraft, or generally speaking aircraft with yoke-type controls, have much larger deflections to steer the aircraft. There, a shaking yoke does not interfere with your controls as much.

To sum it up: it is not necessary and would do more harm than good.

Would some sort of haptic-feedback system (like those built into most video-game controllers) be able to provide the pilot with a tactile stall warning without generating spurious flight-control inputs?

That's difficult to tell. It would have to be strong enough to be noticeable, but not too strong as to not interfere with the stick movements. My guess is that you will probably not be able to satisfy both requirements. The aircraft is really sensitive to stick inputs, especially in alternate law or abnormal attitude law.


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Why do the C-141 and C-5 have T-tails?
Both the C-141 Starlifter and C-5 Galaxy have T-tails, where the horizontal stabilizers and elevators are mounted on top of the vertical fin rather than on the sides of the aft fuselage.  T-tails are usually only found on aircraft where the real estate on the sides of the aft fuselage is taken up by engines (Il-62, 727, DC-9, Tu-154, etc.), as the T-tail configuration has the serious disadvantages of

needing a vertical stabilizer strong enough to carry the entire weight of the horizontal tail plus the considerable downward aerodynamic forces generated by same, making for a considerably heavier (and, thus, less fuel-efficient) airframe,
causing maintenance difficulties, as it's harder for the mechanics to inspect and repair something that high up (this was an indirect cause of the crash of Alaska Airlines Flight 261), and
being susceptible to an extremely dangerous "deep stall" condition, where, at an angle of attack slightly past the stall point, the disturbed wake from the aircraft's wings blanks out the horizontal tail, causing a total loss of pitch control and making recovery essentially impossible.

The C-141 and C-5 have conventional wing-mounted engines, yet both have the tail configuration of an aircraft with fuselage-mounted engines; why is this?

It comes down to a matter of philosophy. Or, if you are less generous, it’s a matter of fashion. Note that the Russian equivalent to the C-141, the Il-76, uses a T-tail, too, while the An-124 uses a conventional tail.
T-tails were in fashion in the early jet age. They were widespread, first in fighters and later in transport aircraft, too, because they:

Have a longer lever arm when placed on top of a swept vertical, reducing tail size and mass,
Allow for a smaller vertical tail due to the endplate effect, which lowers the requirement for hangar doors and ceilings. In case of the A400M, the T-tail allowed for a height reduction of approximately 2 meters.
Stay in undisturbed flow during stall. With wing-mounted engines resulting in a more forward wing position, the danger of a deep stall is reduced when compared to a configuration with tail-mounted engines.
Allow for less interference drag, especially when combined with a Küchemann body in the intersection between vertical and horizontal tail.

Therefore, if your conceptual design team is dominated by aerodynamicists, it will tend to favor the T-tail version. Only when second-order effects are considered will the conventional tail win. And those second-order effects will only become known by experience.
What are those effects?

Flutter! The rear loading ramp requires cutting the rear fuselage open, reducing its torsional stiffness. Add to that a mass on a long lever arm (the horizontal tail) and you get a structure with a very low eigenfrequency. In order to get that frequency out of the usable speed range, you need to stiffen the fuselage,
which adds structural mass. Both tail surfaces might be smaller for a T-tail, but the whole aircraft will be quite a bit heavier. During flight test of the C-141 it was found that the antimetric wing bending mode would nicely couple with the torsional Eigenmode of the the tail, resulting in some scary footage. Also, a coupling between elevator rotation and horizontal tail torsion required to generously add mass balance to the tail.
And then there still is this lingering suspicion that you might get into a deep stall. However, with the long distance between wing and tail of a configuration with wing mounted engines this is less of a threat than it might seem.
A real problem, however, is the tendency of a T-tail to produce a strong nose-down pitching moment in a sideslip. A low tail will eliminate this effect.

And the added structural mass from stiffening the fuselage is on top of (so to speak) all the extra mass added to the vertical stabilizer to allow it to support the weight of the horizontal tail and the aerodynamic forces therein.

High wing military aircraft (c141 and c5 both are) use T tails for multiple reasons.

to prevent rocks and debris from damaging the tail when landing off airport
to keep tails out of the engine thrust line (which are higher than on a low wing aircraft) during cruise where vibration may cause fatigue
a T tail can be smaller (lighter with less drag) as the horizontal stabilizer is located farther behind the fuselage and the vertical stabilizer has an "endplate" which increases its effectiveness.

Deep stall is not the same problem a low wing aircraft has with a T tail. On a high wing aircraft with a T tail the horizontal stabilizer is in approximately the same place vertically relative to the wing as as it would be in a standard empennage on a low wing airplane, and so is no more susceptible to wing blanking in a stall.

Would it not also make it easier to unload cargo from a rear ramp?

@jamesqf Possibly, but the c130 has a conventional tail and doesn't have a problem with unloading cargo.

A primary reason is that it facilitates a large cargo hatch and ramp to be built into the rear of the plane, for loading and unloading. No elevator spar running through the middle of the rear of the aircraft to get in the way.
Note that both the current C17 and A400m transports, very recent designs, also use a T tail. Both have large cargo hatches in the rear.
True that the C130 doesn't use a T tail and has a large cargo hatch in the rear, but it's elevators are built very high on the fuselage, and this design was not carried forward with the later C5, C141 or subsequent large military transports. (C130 is a 1950's design) Also, the C130 tends to travel at lower airspeeds than the C5, C141, C17 and A400m, so there may be an aerodynamic factor, though I can't find any hard information one way or another on that.

The elevator spar has plenty of space in the area occupied by the rear ramp door when it is folded up into the fuselage. See the C-130 and the whole Antonov range for example. The wing spar is a much bigger obstacle; see the different cargo hold heights before and aft of the wing spar. And no, there is no speed-related factor.

I worked on both aircraft for Lockheed (1967-1983). The reason for the T tails is a government requirement that the aircraft must be able to land in a depression or say wide ditch. The stabilizer must clear the required depression height. True I am 80 years old but my mind is clear enough to remember this. We did a tremendous amount of time in flight test checking stalls because of the T tails. Those were the days.

That is an odd requirement: when you put either aircraft in a ditch, the engines will hit the ground long before a fuselage-mounted tailplane. Or was this with a more specific requirement in mind?

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What determines the maximum speeds for a particular aircraft?
Aircraft have three different maximum airspeeds:

VMO (maximum operating speed) - the maximum speed that the aircraft is allowed to reach during normal operations.
VNE (never exceed speed) - the speed that should never be exceeded during operation, even in an emergency, as flying at speeds higher than this could damage the aircraft.
VD (diving speed) - the fastest they got it to fly during testing.

What part(s) and/or characteristic(s) of the aircraft determine the values of these three maximum speeds?  Is it that the plane's engines are incapable of pushing it through the air any faster?  Would the wings snap off from excessive aerodynamic loads if it flew faster?  Is it something else entirely?

Actually, aircraft have many more. Of those I can think of, Vfe and Va both definitely count as maximum airspeeds to me.

Maximum flap deployment speed, Maximum gear deployment speed, Maximum window opening speed. Turbulent air speed, Speed to enter a maneuver; both often have weight limits as well. Going too fast can damage the various parts.  I don't know if they are calculated design limits, or if they were determined experimentally. Maybe a combo - calculate a speed, and confirm at a speed a  little lower that nothing is damaged and then post that as the speed.

A quick list:

Maximum speed with flaps at maximum deflection (v$_{FE}$)
Maximum speed with flaps at take-off setting. Fly faster and the flaps might be torn off.
Maximum speed with gear down (v$_{LE}$). Here not the gear itself, but the open gear doors will fail if that speed is exceeded.
Maximum speed for full control deflections (maneuvering speed, v$_A$). Yank the stick to full deflection at a higher speed and risk overstressing the structure.
Maximum speed in gusty weather (v$_B$). Here it is assumed that the aircraft happens to fly into a vertical gust of 50 (FAR part 23) or even 65 (FAR part 25) ft/s, which will greatly increase the lift force on the wings. Above that speed such a gust will overstress the wings.
Maximum operating speed (v$_{MO}$)
Design cruising speed (v$_C$)
Maximum dive speed (v$_D$). This is the speed reached when at v$_C$ the aircraft enters a shallow dive and the pilot needs 20 seconds to react.
Maximum operating Mach number (M$_{MO}$)
Design cruise Mach number (M$_C$)
Maximum dive Mach number (M$_D$). At least 0.05 Mach above v$_C$.

And that is only what is listed in the certification regulations. You can also distinguish between a maximum speed at which the landing gear is operated (v$_{LO}$) and the aircraft is flown with the extended gear, since large doors open during operation but are closed again when the gear is extended. And yes, for aircraft with retractable landing lights, there is even a dedicated maximum speed at which the landing lights are operated (v$_{LLO}$). As usual, Wikipedia even lists a few more.
Aircraft have several limits, depending on altitude. At low level, the dynamic pressure limits how fast they can go, mostly because of structural loads. At high speed, a deflected aileron will twist the wing, reducing its effectiveness. Better limit the speed to a point where the aileron is still effective.
Higher up, it is the Mach number which might limit your speed. If the aircraft is not designed for transsonic flight, Mach tuck and buffeting will set a clear limit, and the maximum Mach speeds are defined to prevent the airplane from venturing into that territory.
Heat loads might pose another limit. The F-16 is limited by the heat loads on its polycarbonate canopy and must not go faster than 810 KIAS at low altitude.
And, of course, there is always flutter. However, for certification it must be demonstrated that no flutter will occur within the allowed speed range +20%.

There are various physical things that will limit the top speed of an aircraft. Which one you will hit first depends on the airframe in question but here are some things you may encounter (aircraft dependent):

Drag will generally counter your thrust which will limit the speed you can achieve in level flight and is often the practical limiting factor on aircraft that actually fly. Level flight is important to note as you can gain a huge amount of speed if you simply enter a dive.
For propellor driven aircraft there are some practical speed limits because of the propellors.
At some point flutter will become a problem and you may literally shake the wings off the plane.
Around the speed of sound (mach 1) you are going to have all sorts of interesting things going on if you are pushing a subsonic optimized airframe to such speeds.
Ultimately, if you still have your engines and wings heat becomes the real problem. The Concorde pumped fuel through the nose to use a coolant in the mach flight regime and was in part speed limited by nose temperature. For manned flight this becomes the limiting factor at the extreme end of the envelope and aircraft like the SR-71 had a very physically demanding environment.

While not an answer or even really related, this answer by Peter Kampf has a really nice video showing flutter and how it is dangerous.

So how do we know flutter won't occur randomly in usual cruising speed?

@Cloud: By testing for it. Thoroughly.

The Main Reason:
Structural integrity of the wing.  The lift equation is listed below:

You can see that the airspeed is the most influential factor for lift since the term is squared.  Also remember that lift is a force so too much lift can rip the wing off of the airplane.  An aircraft wing is only rated to take so much force before the structural integrity fails.

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Why can’t the newest high-bypass turbofans run at their EPR, temperature, and RPM limits simultaneously at sea level?
The General Electric GE9X turbofan (used on the 777X), unlike the vast majority of turbofans before it, did not undergo the traditional triple-redline test (where the engine is run for a prolonged period of time while simultaneously at redline EPR, redline temperature, and redline RPM), because it is, apparently, incapable of doing so except at high altitude:

... Before certification, final tests include a full durability block test, replacing the usual “triple redline” test at maximum temperatures, pressures and speeds, as modern high-bypass ratio engines cannot achieve all maximum conditions near sea level...

In contrast, the GE9X’s immediate predecessor, the GE90 (used on some first-generation and all second-generation_777s), did undergo triple-redline testing before entering service, and again before the release of the high-thrust variants used on second-generation 777s.1
One would expect, intuitively, that it would be easiest to achieve triple-redline conditions at sea level, as fuel and air flow (and, thus, RPM, EPR, and thrust) are highest in the dense low-altitude air,3 and the increased cooling from the additional airflow should be just about balanced by the increased heat generation from the additional fuel burned.4
What makes it impossible for very new turbofan designs to reach all three redlines simultaneously at sea level?

1: Setting a record for the highest thrust ever generated by a jet engine (569 kN, or 64 short tons for you Americans) in the process, which stood unbroken until it was eclipsed by a 597-kN (67.2-short-ton) GE9X test run in November 2017.2
2: Ironically, while the GE9X is larger and has a higher record thrust than the high-thrust GE90s, and is required to power an aircraft just as heavy (and soon to get much heavier still, with the 777-10) as those using the latter engine, its rated thrust is considerably lower (470 kN [52.5 short tons] versus 510 kN [57.5 short tons])!
3: Although not by very much, as the maximum thrust available from (and, thus, fuel and air consumed by and RPM and EPR attained by) a turbine engine remains nearly (although not completely) flat up to very high altitudes.
4: These effects would be even more pronounced on an actual aircraft than in a test stand, due to the higher thrust settings needed to counteract the increased drag from low-altitude flight and the increase in airflow through the engine (and, thus, in fuel flow, as well) resulting from the ram effect of the aircraft’s (and, thus, the engine’s) forward motion through the air.

My guess is, at sea level air is dense and the fan can not spin fast enough then LP turbine generates too much back pressure and chokes HP turbine.

might it be to keep the chamber pressure on ground lower, giving a lighter engine?

The engine has just one control. That is not sufficient to tune three different parameters. You need two more parameters. Those parameters are inlet pressure and velocity. That means the engine will hit all three redlines simultaneously only at specific altitude and speed—which might or might not be possible to simulate on the ground with testing equipment.

@JanHudec what about variable-pitch stators?

@pericynthion, variable guide vanes are not an independent control. They need to follow their schedule to maintain angle of attack on the rotor blades within working range and can't add much resistance to the engine.

What makes it impossible for very new turbofan designs to reach all three redlines simultaneously at sea level?

The implication in your question is that GE9X was not able to reach all three redlines simultaneously but that previous engines (e.g. GE90, GEnx, LEAP, etc) could.  That is incorrect.  Those engines could not reach triple redline either. At least not, type design unmodified.  The engines that were used to demonstrate compliance to FAR 33.87 for those engine programs were modified from their type design in order to be able to reach triple redline. Very heavily modified.
To understand why, let's talk a little about engine performance.  The three variables of N1, N2, and EGT are controlled by several conditions: the throttle setting, the ambient temperature, the ambient pressure (i.e. altitude), the forward velocity, and the deterioration of the engine (e.g. how worn out the seals are).  Now, if we are doing this test on a test stand (as opposed to flying it), then we really only have control over one variable: the throttle.  Everything else just is what it is. It shouldn't be too hard to see that you can't control 3 variables precisely with just one knob (E.g. imagine in your bathtub or shower you try to control both the water temperature and water flow rate with just one knob. You can't do it.  To control 2 variables, you need 2 knobs).
More specifically, considering sea-level static conditions, if the ambient temperature is cold, then the air is dense, so the fan pumps a whole lot of mass with every revolution and makes a lot of thrust.  You have to burn a lot of fuel to do that.  So you'll hit EGT limit first before you get anywhere close to N1 redline.  If the ambient temperature is hot, then the fan doesn't pump as much per revolution, so speed needs to be really high in order to make any kind of thrust.  You'll hit N1 redline before you hit EGT redline. You can presume that there should be one magic temperature where you could hit both of those at the same time, but if you just sit around and wait until the weather hits the exact right temperature you wanted and stays there for 150 hours (the length of the test), you'll be waiting a long time.  And even then, you still won't get to N2 redline at the same time.
Ok, so how would you run a triple redline test then, given that we can't control ambient temperature? Well first you'll need to way to vary the thrust the fan produces.  E.g. you could use a variable fan nozzle.  On a cold day, you open up the fan exit area so you can get to higher N1 speeds with less thrust, and on a hot day you throttle it down.  There are other options but this is easiest to explain.  Then you need to control N2.  The easiest way here is to open or close the variable stator vanes.  This has the effect of making the HPC do more or less work per revolution, so you can tune in the exact speed you want.
Ok, so we added a variable fan nozzle and a different VSV schedule, and we are at triple-redline.  Problem solved right?  Wrong.  Jet engines are extremely complicated devices with all kinds of interdependent requirements.  If you change one thing, you end up affecting other stuff.  Changing the VSV schedule might introduce aeromechanics issues onto the HPC blades, or change the cooling flows to different portions of the turbine.  So then you have to make other modifications to the engine, like changing the sizes of all of the different cooling pipes so you get back to the original flow rates you wanted.  But then, those changes also have other effects, which snowballs.
Historically (I mean like 1970s 1980s) the engine companies could make a few small changes and hit triple redline.  As the engines became more and more complex, more and more changes were required. At some point, the list of changes required to make the engine hit triple redline is so big that the thing you are testing is a crazy chimera of an engine that has only a faint resemblance to the actual type design.  At that point, running a triple redline test no longer makes sense.  That's what happened with 9X. Therefore, GE found an alternative way to satisfy the endurance test requirement.

If the engine is inside a test cell, you don't need to wait until the temperature, pressure, etc., are just right - you can vary the atmospheric conditions inside the test cell until they're just right.

@sean, for very small engines, like a military fighter or business jet, test cells do exist that can control atmospheric conditions as you mentioned.  However, the mass flow rate of a GE9X engine is something like 1.5 tons per seconds. A test cell that could produce that much flow at an arbitrary temperature and pressure would be exorbitantly expensive. None of the major engine manufacturers have anything close to that size.  Biggest is maybe 1/10th of the required mass flow.

@DanielK You're underestimating the U.S. Air Force. AEDC regularly tests large commercial engines in ASTF, which can indeed vary pressure and temperature. I'm pretty sure the Trent 900 was tested there, for example. GE9X would indeed probably be a bit past its capabilities, but not anywhere remotely close to 10x. More like maybe 1.1 to 1.3x. So, yeah, the engine manufacturers might not have such facilities, but they can rent Air Force facilities that do have those capabilities.

According to AEDC's website, ASTF can test pressure altitudes up to 75,000 feet for engines rated for up to 100,000 lb thrust. That's indeed a bit below the GE9X, but plenty for basically any commercial engine other than the GE90/GE9X. Oh, and it can do that at Mach 2.3.

@reirab Interesting. I see that the mass flow rate is 1500 lb/s.  That's too low for GE90/GE9X by a factor of 2.  But a mid size engine like a LEAP or PW1000G would fit. Thanks for pointing that out.

@DanielK It's definitely far too small for a GE90 at sea level, but it's not that far off for high-altitude testing. But you don't really need that level of facility for sea level testing anyway. Just an open-air sea level test stand will do for that. They have those, too, though I think they're mostly just used for military engines, since the engine manufacturers have their own sea level stands.


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Does the FAA prescribe radar cross-section requirements for civilian aircraft?
Are there any FAA regulations stipulating the minimum permissible radar cross section for civilian aircraft (i.e., making sure that civilian aircraft are at least such-and-such cross-section visible to primary radar)?

No. Primary radar isn't a universal tool to begin with. Secondary radar and thus transponders are more commonly used. So there are regulations as to where and which aircraft need to install transponders. Ultralights and some GA can get away without.
Related: Is it legal to make a non-military stealth plane?
No RCS requirement, but you'll need a transponder regardless.

I would imagine a Cirrus's natural radar cross section is pretty small with only the engine and minor metal bits to reflect.


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Why are there extra restrictions on constant-speed propellers?
Constant-speed propellers are far more efficient than non-constant-speed propellers, allow the engine to be made considerably simpler (due to removing the need to continuously adjust the ignition timing based on how fast the shaft is rotating at any given moment), and, unlike traditional variable-pitch propellers, don't require the pilot to manually adjust the propeller pitch.
However, according to Wikipedia, propellers with constant-speed mechanisms have considerably tighter restrictions on them than other propellers:

A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU [constant speed unit; the part of the propeller mechanism that makes it a constant-speed propeller]. CSUs are not allowed to be fitted to aircraft certified under light-sport aircraft regulations in the United States.

Given that a constant-speed propeller performs its pitch adjustments automatically, without the need for pilot intervention, and that constant-speed propellers perform better, not worse, than other propellers, why all these extra requirements and restrictions?

Constant speed propellers have 3 knobs to control the engine/prop (Prop Pitch/Throttle/Mixture) where fixed pitch props only have 2 (Throttle/Mixture). Some aircraft simplify the CSP controls (pitch/throttle) into a single lever, but either way it is helpful to the pilot to know what to do if your propeller starts to run away.

@RonBeyer: I thought one of the big advantages of having a constant-speed propeller rather than a vanilla variable-pitch propeller was that the propeller pitch adjustments were performed automatically rather than manually (like how a car with an automatic transmission is more efficient and easier to drive than a stickshift)?

No, it is not "automatic" in that it does it all automatically. The pilot still needs to set the prop speed, engine speed (throttle) and mixture. The automatic part is the prop maintaining that speed, it still has to be set.

@RonBeyer, slight correction: the throttle sets engine power – the speed is obviously the same for engine and propeller (well, for turbine engine with free power turbine not exactly, but they are still quite closely coupled there).

@JanHudec Of course, but we all call it "throttle" :)

@Sean, “vanilla variable-pitch propeller” is essentially a curiosity. It never became common, because the constant-speed system is just a couple more parts and pretty obvious.

And aircraft with constant speed props have another additional complication, the ability, (need) to feather the propeller in the event of engine failure, (and understanding the consequences of feathering).

@CharlesBretana That is a little too broad. My CSP Cessna does not have the ability to "feather" the prop in the case of engine failure, it uses the engine oil pump to drive the pitch control, no oil pump, no pitch control (it fails at low pitch/high RPM).

@Ron, thanks, did not know that. I thought all constant speed props had a feather capability. It's a good day when I learn something new!

@CharlesBretana The biggest reason to have a feathering prop (at least on twins) is to minimize drag on the non running side, especially if you lose the critical engine. Other CSP's can feather just because it has other benefits, like ground beta, there is only one light single I know of that can go into beta and also feather (other than turboprops).

@Ron, Well, feathering reduces drag on a single engine aircraft as well, improving their engine out glide ratio and resultant range and time available to deal with the emergency. Granted, by a small amount, but if you already have a constant speed propeller, adding the ability to feather it probably adds very little to aircraft gross weight. I could not hazard a guess as to how much more it might add to cost.

You might go from 12.5:1 to 12.6:1 for the glide in a single, a couple propeller for my 177 costs $13k without the feathering possibility... I just replaced ours last year.

@RonBeyer: If you go from 12.5:1 to 12.6:1, then, for instance, if you have an engine failure at 2000 feet, you glide for 25,200 feet instead of 25,000 feet.  That's a difference of 200 feet, which could potentially be the difference between clearing a bunch of trees and flying into them, or between making a runway and coming down in the canal just short of it.

The engine is the same, not simpler. There is one additional level of complication to the system -- the constant speed propeller has a number of extra parts.
The pilot needs to handle one more lever, selecting the speed. And needs more training in order to remember to put it at the right setting in various flight conditions -- not to mention how to handle an emergency when the constant speed unit falls (you need to prepare for every possible situation).
All in all, more training and more complicated systems sets up the requirements for restrictions.

And don't forget there is also "variable pitch" still used in Bananza, Navion, Grummon Goose, etc on engines like the Continental "E" series that is more labor intensive than a constant speed. A "variable pitch" is a constant speed without a governor.

Without a FADEC that is, right?

@Raffzahn: agreed. With a Fadec there can be a single power level control for both engine and propeller. Diamond Aircraft and Piper are examples for new aircraft, I believe used with A1-burning diesels. But the majority of the fleet is still "old-style" with three controls. I am not in the loop to know how that effects training and requirements.

@ghellquist AFAIK not at all, at least not regarding FAA pov.

I recently moved from a fixed pitch propeller (FPP) (cessna 152) to a constant speed propeller (CSP) (Piper Dakota). In the FPP scenario the engine RPM is controlled by the throttle only. High throttle means high RPM and high power/thrust. In the CSP scenario the engine RPM is controlled by the throttle as well as the propeller pitch. One important thing to remember while flying is that you don't want to redline the engine RPM. Thus various power settings are defined in the power settings table in the POH. Also, when you come in to land you need to remember to push the propeller back so that you will have maximum power available if you need it ( in case of an abort ). All this means that you need extra training to use the propeller control effectively and not to damage the engine in the process. In reality, if you want to go fast cruise at full power at a high enough altitude ( say 7500 ft), then you probably won't even need to touch the propeller control ( this is true for Piper Dakota, I can't say for other planes)

A pilot requires some additional training and, in most jurisdictions, a formal signoff before being allowed to fly aircraft fitted with a CSU

What jurisdiction is that? Who is the formal signoff satisfying?  In the US, I went from flying a Cessna 150 with fixed pitch prop and 100 HP to flying a Cessna 177 with constant speed propellor and 180 HP, also fixed gear. No formal signoff was needed to satisfy the FAA, nor the insurance company that I used to insure the C177.
If I had gone to 200 HP, retractable gear, and constant speed prop, then a complex and perhaps high performance rating would have needed to be signed off.
Also the insurance company would likely have required some number of hours to be flown with an instructor before being allowed to fly the plane solo.
So, just the constant speed prop alone should not be a driver for a formal signoff.

Good point. A lot of people think you need an endorsement because they got one when they learned to fly a constant speed prop. When I got my I got my high-power endorsement, I learned to use the constant speed prop at the same time. Likewise, most complex airplanes also have a constant-speed prop, so people getting their complex endorsement conflate the requirement.

There are engine operating rules which govern the manipulation of throttle vs prop.
Most commonly.
When increasing power:
1) Increase Prop RPM, first.
2) Increase throttle, second.
Then the opposite for decreasing power.
This order of procedure is to protect the engine from preignition/detonation due to a sudden increase in Manifold Pressure at a low rpm.
When these restrictions were originally implemented, many engine manufacturers had models utilizing gear reduction of the prop, where the engine turned faster than prop rpm. Continental had an engine with a full 2:1 reduction. If the pilot used the more common rule for power increase/decrease, gearbox damage would occur as a result of sudden unloading and even negative torque (Backlash). In aircraft with these engines the procedure was the opposite.
When increasing power:
1) Increase throttle, first.
2) Increase prop rpm, second.
Then the opposite for decreasing power.
Going back a little further. Aircraft fitted with Hamilton Standard "counterweight" props had to be shut down in max pitch, so that oil would not cool and thicken inside the prop, pitch-change cylinder as it cooled.
A lot to remember for someone just transitioning from a fixed pitch prop, back in the day. Maybe there wouldn't be as much of a need for the restrictions in today's equipment.


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Are there any known instances of airline pilots passing off aerobatics as “turbulence”?
Inspired by this Far Side cartoon, which shows the pilots of (presumably) a passenger airliner rolling the plane steeply from side to side, and claiming (to the passengers) that they’re just passing through some turbulence.
Are there any known instances of airline pilots actually doing something like that during revenue flights?  Yes, I am perfectly aware that this would be considered highly unprofessional behaviour, and, as we all know, pilots never behave unprofessionally.

I have a second hand story from a reliable source (crew, family) who tells of at least one instance of this.  I sincerely believe it's true.

Recent discussion with friends who work or worked for one  major US airline, say they never witnessed such a thing.
One of them points out that on Navy P3 flights, they would do it to newbies. Not often, but sometimes to recalibrate newbies.
With respect to commercial flights, all pointed out that flight attendants are at high risk for injury, and would not tolerate such actions. All it takes is one person getting hurt. A neck injury from an unsecured passenger, or a burn from hot coffee happen often enough on flights with "natural" turbulence.  Creating turbulence could be career ending, and could have civil liability.
The population of pilots I was dining with are all over 45, and tend to be rather risk adverse. One, who has been retired for quite some time (he's in his 80's) says that people joke about it, but in all his experience, he too has never witnessed "pilot induced turbulence."
I have a blow-up of that far-side at my desk, though...

Another point is that (at least from my limited experience) aerobatics aren't all that rough.  For instance, if you couldn't see outside during a loop or roll, you might not notice much, while turbulence can have you banging your head on the cabin roof in spite of being belted in.

If Bob Hoover did it you wouldn't even spill your tea!


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How is radar separation assured between primary and secondary targets?
Air traffic control radars come in two flavours, with most ATC centers using both and displaying the output of both on a single screen:

Primary surveillance radar (PSR) is a true radar system, which detects aircraft (and also non-aircraft objects, such as birds, balloons, weather, parts detached from aircraft, the ground...) by bouncing radio waves off them.  PSR is valuable because it can detect not just aircraft with functioning and responding transponders, but also aircraft without functioning transponders and the aforementioned non-aircraft-but-potentially-of-interest-to-or-regarding-aircraft objects.  However, in order to maximise the amount of airspace covered per 360º sweep, the radar beam from most civilian PSR transmitters is fan-shaped; at any given moment, the beam covers (for all intents and purposes) just a single azimuth, but is spread out over a wide range of elevation angles.  The radar has no way to determine the elevation angle of a returning reflected signal, which makes most PSR systems useless for discerning altitude, and introduces some imprecision and a variable amount of inaccuracy into the ground position of a target (as the object producing the return could be located anywhere along a vertical arc segment at the target’s slant distance in the plane of the radar beam).1
Secondary surveillance radar (SSR) isn’t, strictly-speaking, radar at all.  Instead of bouncing radio waves off the local flying-object population, it, in essence, calls out “Is anyone there?” in the blind; transponder-equipped aircraft hear this call, and the aircraft’s transponder then broadcasts the aircraft’s position, altitude, airspeed, direction, flight number, aircraft type, licence, registration, proof of insurance, etc., allowing the SSR system to plot the exact position of the aircraft generating the return, along with the aforementioned other various interesting pieces of information.  This makes SSR extremely useful, especially in congested airspace, but it can’t detect anything without a transponder, or with a broken transponder, or with their transponder turned off.  SSR is planned to eventually be replaced by ADS-B, which works in essentially exactly the same way, except that the aircraft’s transponder broadcasts all of the aforementioned information in the blind, without being asked, thus eliminating the need to have a large, expensive SSR transmitter for each ATC facility.

Here’s an illustration of the difference in the determination and display of target positional information between primary and secondary radars:

Separating secondary targets from each other is (assuming that the air traffic controllers and pilots concerned are paying attention and doing what they’re supposed to), obviously, trivial; you just need to avoid having two or more aircraft occupy the same position at the same altitude at the same time.  Separating primary targets from each other is also fairly trivial, despite the fuzzy positional information and the lack of altitude information; two or more aircraft occupying different vertical arc segments (and, thus, separate from each other) will show up as two or more separate targets, so the trick is to keep two or more aircraft from occupying the same arc segment.  If you’re dealing with just primary targets, separation between aircraft can be assured by keeping the dots on the screen away from each other.
If, however, you’re dealing with a mix of primary and secondary targets, I see a problem arising: since positional information is determined differently for primary and secondary targets, an aircraft in a given position could show up in one of two different places on the screen, depending on whether it’s a primary or a secondary target:

It should be immediately obvious that the same effect could be produced if the primary and secondary targets were produced by different aircraft; two aircraft in dangerously close proximity to each other could appear to the controller to be separated by a safe distance, if one of the aircraft is a primary target and the other is a secondary target.
How to solve this problem?

1: Some PSR systems (primarily military systems, as it’s much easier to vector aircraft to intercept an intruder if you know its altitude and its exact location) do provide altitude information; some of these work by using a narrow “pencil beam”, which allows the determination of a target’s elevation angle at the cost of greatly-increased scan times, while others use a pair of radars at different heights and measure the difference in slant distances to the target between the two radars, allowing the target’s altitude to be determined using trigonometry.

Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.

Related question: How do ATC radars know where to position a primary target?

@GregHewgill: SSR can work as shown in the figure -- that is, with Mode S the interrogator can ask for a GPS position report in the same format used by ADS-B.

Interesting podcast about SSR

I can't speak to how ATC radars operate in practice, but mathematically it's pretty trivial to convert a lat-lon-alt coordinate from a secondary target to a range-az in the primary coordinate system.

Show thy face, downvoter...

You say that "SSR isn’t, strictly-speaking, radar at all." I've mentioned this elsewhere on the site, but if you blur your eyes a little then SSR and ADS-B could be termed "radio ranging." Terming them "radio detection" is somewhat more of a stretch but not completely laughable.

Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.
ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.

Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.
The ATM system does its best to avoid displaying multiple targets for the same aircraft.

I was told that separation with primary targets is usually not assured.
The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.
So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).
Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.

Usually the PSR and SSR radars are co-located.  A plot combiner at the radar site combines them (or not), and transmits reports to the ATM system as SSR reinforced, SSR only, or PSR only.
Then, expanding somewhat on @Greg Hewgill's answer, the ATM system receives input from multiple radar sites, and possibly other sources such as ADS-B.  In most of the developed world except for extremely sparsely populated regions, aircraft are usually within the coverage of multiple radar sites, which may be tens or even hundreds of miles apart.  Since the earth is curved, the "horizontal" plane at one radar site is different to that for a radar 200 miles away.  The positions must not only be slant-range corrected but also converted to a common "system plane" so that position reports for the same aircraft from different radars can be correlated to the same system track.  Occasional missing SSR data from one radar can usually then be obtained from a different radar (except of course in cases such as transponder failure).

A transponder does not, in fact, transmit the plane's position, or altitude, or direction of flight, or any of the other things you said. The standard civilian transponder only transmits two or three pieces of information:

The 4-digit transponder code entered by the pilot.
Whether or not the pilot has pressed the "ident" button in the last 12 seconds.
The current barometric pressure as measured by the static port (only for Mode C).

Position information is derived from the transponder reply the exact same way as it is for primary radar: The direction is determined based on which way the dish was facing at the time it sent the interrogation, and the distance is based on the time it took for the reply to come back. This means that the slant error is exactly the same for primary radar as it is for the secondary.


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Why would one want to slow to V2 following an engine failure during takeoff?
Reading about the crash of American Airlines Flight 191, the proximate cause of the final loss of control was the pilots' reduction of the aircraft's airspeed to V2, in compliance with the published procedures for a single engine failure after V1 during takeoff in a DC-10-10; unbeknownst to the pilots, the damage caused when the #1 engine separated from the aircraft had caused the left wing's outboard slats to retract, decreasing the left wing's stall angle, with the result that decelerating to V2 required the aircraft to fly at an angle of attack at which the left wing stalled.
But why would one even want to slow to V2 in the first place?  Even if we, like McDonnell Douglas, ignore the possibility of an engine separating from the airplane and damaging it to the process, there are still good reasons to go faster, not slower:1

If an engine has failed, there's no way to immediately know whether it's a contained or an uncontained failure; if it's an uncontained failure (which is, unfortunately, a common enough occurrence to demand consideration when designing an aircraft), there's a very good (or bad) chance that the exploding engine (especially if it's a wing-mounted engine, like the DC-10's #1 engine) has damaged the aircraft's flight control systems, in which case you'll want to be flying as fast as possible in order to maximise the control authority of the surviving controls.
If there's a chance that whatever caused the first engine to fail could also take out the remaining engines (for instance, contaminated fuel or a flock of birds), you'll want to get as much speed reserve as possible, so that, if the remaining engines do fail, you'll be able to zoom-climb and reach your best-glide speed at the highest possible altitude, thus giving you the most time and space to do something.

What good reason could there possibly be for eroding your already-degraded safety margins yet further by slowing to V2 following an engine failure?

1Not that doing so was justifiable of their part (it wasn't, as the NTSB accident report makes clear).

I never flew the DC-10, but my first reaction to your question was that if I have airspeed in excess of V2, I'm not climbing as fast as I could. Looking at it from the standpoint of energy management and considering whether to have more speed or more altitude, my gut reaction is that I want the altitude. I can always convert altitude to speed (and altitude is good from the standpoint of obstacle clearance). As you note the extra speed would enable a zoom-climb, but the time spent at the extra speed would have meant more drag than at V2 and that difference would not be recoverable.

Simply put, V2 is the speed that guarantees the best angle of climb on one engine. Any slower or faster increases the risk of hitting an obstacle.
Drag
There are 2 main categories of drag - induced drag, and parasite drag. Induced drag is an unavoidable consequence from lift and largely shows itself in the form of wake vorticies. Induced drag is largest at high angles of attack - i.e low speed.
Parasite drag includes drag from things like skin friction, rough surfaces, wires, and so on. Parasite drag is largest at high speed.
If you plot both categories of drag together, you end up with a chart like this:

The aircraft must provide enough thrust to overcome the total drag, so this can be renamed the thrust required curve. If you picture a horizontal line across the top of the graph, that would represent the thrust available for a jet engine. The speed which provides the biggest gap between thrust required and thrust available, or excess thrust, is the speed at which the aircraft climbs at the steepest angle.
But if you lose 50% of the thrust available, then that horizontal line moves down a lot. If you are going too fast, the total drag might be more than the available thrust, forcing the aircraft to descend.
That's why V2 exists, to give the pilots a guarantee that at that speed, there will still be enough excess thrust to climb at the steepest possible angle.
It's true, slowing to V2 doomed this flight. But uncontained engine failures, especially ones that damage the wing, are extremely rare, and more rare than that is a dual engine failure for whatever reason. The powers that be decided it is safer to give pilots the V2 target so they don't mess up in an already high-workload scenario and hit a tree.

So V2 is just another name for Vxse (or whatever the equivalent to that is for aircraft with more than two engines - Vx[n-1]e?).

V2 by definition is the speed an aircraft will climb with an engine failure.  Notice this does not mean further reducing power.  The aircraft will pitch to maintain V2 as per procedure with whatever thrust it has.
The crux of the question is how close V2 is to stalling speed in the event of a multiple malfunction.  In this case the slats retracted when the engine separated.  This was probably an unforeseen scenario.  Procedures and design certainly warrant review.

I wouldn't that much worry about the stall speed, rather the Vmca speed.

The modified engine change procedures devised by the airline to reduce maintenance man-hours (with increased risk of damaging the pylon-to-wing attachment points) certainly needed review, since they were the root cause of the accident!

@VladimirF: You should, given that the stall speed is ~30kts above the (purely theoretical) V-MCA.

authors: Vladimir F Героям слава


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Why don't aircraft carriers have multiple sets of arresting wires?
On this answer to this question regarding the merits of angled flightdecks on aircraft carriers, @JanHudec makes this interesting observation:

You don't really have "two flight decks". The catapults are well distributed, but there is just one set of arresting wires, so while hit to the bow disabling catapults 1 and 2 would leave 3 and 4 available, hit to the landing area still prevents all landings.  [Emphasis added.]

Why don't aircraft carriers have multiple redundant sets of arresting gear, to allow them to land aircraft even if one of the landing runways is disabled by battle damage?

If you’re talking about multiple landing areas (LAs), the idea has been considered by the Navy; early concepts of the Ford-Class carriers were large catamaran type vessels featuring parallel LAs.  But the designs were excessively complex and expensive and not likely to survive to maturation.
Laying out additional arresting gear on the angled LA would present some difficulties in that an aircraft needs about 100-120 yards (300-360 ft) or so of rollout for the arresting gear to stop the aircraft without imposing excessive loads on the aircraft or flight crew.  This requires increasing the length of the LA, increasing cost and complexity.
In general, flight ops often have alternate plans in place in the event the ship’s arresting gear cannot recover returning aircraft.  Aircraft can trap aboard other carriers operating in the area (which happened during Desert Storm in 1991) or divert for tankers and/or land based airfields, fuel permitting.


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How does one tell whether a corkscrew approach is stabilized or not?
A corkscrew approach is an unusual type of approach used in places like BGW/ORBI to minimise an aircraft's exposure to antiaircraft fire on the way down; it involves staying at cruise altitude until one is directly over the airport, then descending to the runway in a fast, tight spiral.
Given that the tight, spiralling descent would seem to make it almost impossible to determine whether the approach is stabilized or not, how does a pilot performing a corkscrew approach make this determination?

I'm not sure why anyone would expect a maneuver like that to be stabilized. Are you asking what the correct technique is?

The corkscrew approach still ends with a mostly normal final for the last 500–1000 ft, doesn't it? The term stabilized approach only applies to final.

Being shot at and stabilized approach seem to be a contradiction (oxymoron)?

The corkscrew approach as Wikipedia describes it is what the military refers to as a “Force Protection” measure to be used when operating in a high threat environment.  It is not a published procedure in the traditional sense of the word, but something that is put in place by the theater commander and briefed to aircrews when driven by the current intelligence assessment of the threat level.
It is not intended to be flown as a precision maneuver like an instrument approach would be.  It is a dynamic, eyeballs outside the cockpit looking for a missile, combat visual descent.  A high rate of speed is maintained until as late as possible in order to make evasive action possible.  Not only is it clearly NOT a stabilized approach, it is actually the exact opposite:  Done in VFR conditions, the aircraft isn’t configured until just before landing.
Contrast this with the definition of a stabilized IFR approach from FAASafety.Gov below:

Stabilized Approach Concept. A stabilized approach is one in which the pilot establishes and maintains a constant angle glide- path towards a predetermined point on the landing runway. An airplane descending on final approach at a constant rate and airspeed travel in a straight line toward a spot on the ground ahead.

There is a lot of additional information available about stabilized approaches out there for anyone interested so I won’t attempt a further explanation about the intent.  By definition, however, a corkscrew approach is not stabilized.
So, back to the original question:  “How does a pilot performing a corkscrew approach make this determination?”  (whether the approach is stabilized or not…)  The answer is that a pilot performing a corkscrew approach isn’t asking himself those kind of questions.  The distinction is completely meaningless.
He/she is looking outside for the smoke trail of a stinger rising from the ground, and working to get the plane to a point where they can pull the nose up to break the rate of descent and slow down, slap the gear and flaps down, and make a tight 180 combat turn to a short final to lessen the exposure time when they are dirty and slow.  The goal is to get on the ground as safely and expeditiously as possible and minimize any time spent “stabilized” on a long straight-in approach.
Stabilized approaches are what tactical aviators fly stateside when practicing for their annual instrument proficiency check.

It is very important to know how much altitude you lose per 360 degree turn.  You essentially
set up your turn at a given bank angle, say 45 degrees and a safe speed to avoid an accelerated stall.  You execute the turn at a lower power setting than a normal steep turn,
resulting in a "corkscrew" descent path through the air.  If your AGL matches your altitude loss per turn plus enough to roll out for short final, it works.
So if my plane loses 1000 feet per 360 turn at a given bank angle and power setting, I would
"approach" at any altitude divisible by 1000, say 10,000 feet AGL to avoid AA. Add 300 feet for roll out, starting corkscrew at 10300 feet AGL and checking altitude every turn.
Note rate of descent is still controlled by power, so by checking each turn (half or even quarter turn) and maintaining bank angle, yes, one could say this was a "stabilized" approach.  Your last turn would be very similar to a standard "base to final".

have you ever actually flown a force protection descent to landing in a combat environment?  I'm curious because except for disagreeing that it meets the definition of stabilized, there's otherwise nothing technically incorrect in your response.  I just feel it lacks a sense of the real purpose...

This is why your response excellently compliments mine.  I was focusing on the technique, rather the flying the "L7" pattern.  You can stabilize a constant turn at a known sink rate, this is your approach to the airport.  The "landing pattern" is a very truncated quarter turn.  As with all landings, stepping through the keys correctly increases the chances of a good landing.  Power controls rate of descent.  No, I am not a combat pilot, but would do everything I could to help get them home safe.

Thanks Robert.  You do espouse a good technique.

I don't see any published charts there for anything that looks like a "corkscrew" approach.  Regardless, each operator has their own criteria for what constitutes a stabilized approach and when the airplane must be stabilized during an approach.
If an approach can't be done while meeting their criteria, then either it won't be authorized, or it will get an exception and require special training.


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Why don't militaries use ram-air parachutes?
Military parachutes for use by humans (both paratroopers and distressed aircrew) tend to be of simple round or (more recently) cruciform design:

(Image by the United States Army, via Wikimedia Commons.)

(Image by the United States Army, via Flickr, via Wikimedia Commons.)
These are simple drag parachutes, generating no lift.  In contrast, sport parachutists nowadays almost invariably use much-more-advanced rectangular or elliptical ram-air parachutes, which generate much lift and relatively little drag:

(Image by John [shebalso] at Flickr, via Wikimedia Commons.)
Military use of ram-air parachutes is essentially limited to display teams.
This seems rather odd, given that lifting ram-air parachutes:

allow for much lower descent rates (and, thus, softer touchdowns - something which would be especially important for paratroopers, who jump with lots more heavy gear than sport skydivers, and ejectees, who are likely already injured, and, thus, tend to have less ability to tolerate hard touchdowns) than drag parachutes, due to their greater aerodynamic efficiency;
allow their users to glide considerable horizontal distances once deployed and can be manoeuvred nimbly, which would:

allow paratroops to be dropped some distance from a target and glide silently in;
allow wind-induced dispersal of a large paratroop unit to be negated;
allow paratroops to manoeuvre to land in a suitable area or to evade antiaircraft fire;
allow ejectees to glide towards territory controlled by friendly forces and/or terrain with the least potential for further exacerbating any injuries to the ejectee.

So why do military users stick with drag parachutes rather than moving to lifting ram-air parachutes?

I don't know for sure but I assume it's because it's easier to get your weapon out of you aren't busy flying the chute, plus remember that these are 18 year old kids with nearly 100lbs of equipment, armor, and supplies. The chutes also make decent cover/shelter.

How long do you want to linger in the air when hostiles are shooting at you?

"allow paratroops to manoeuvre to land in a suitable area or to evade antiaircraft fire;"
Show me proof that you can evade bullets simply by changing the shape of your parachute, then I agree with you.

"allow paratroops to be dropped some distance from a target and glide silently in" I'm sure some military branches do have experience with this. Usually those are the branches that take less photographs of their manoeuvrers.

@Mast They're out there if you know where to look. There are some videos of US Marine Corps Recon troops practicing it on youtube.

I'd just like to add something that I don't think warrants a full answer.  I couldn't find public domain pictures easily, but image search 'paratrooper deployment'.  The point of these is to land large numbers of troops.  The air can be pretty densely packed with troopers and their parachutes.  Do you want 1,000 18 year old kids deployed in close proximity to all be steering their own highly maneuverable parachutes?

@EarlGrey: Dodging bullets, no.  Making it harder for the AA gunners to aim in the right spot, yes.

@Vikki That's generally why they do tactical airstrikes prior to airborne operations. They do NOT do mass tactical drops onto defended objectives without suppressing those defenses first. To do otherwise would lead not just to the loss of the paratroopers, but multi million dollar transport aircraft as well.

@EarlGrey it takes a certain amount of time to aim and then for the bullet to fly.  If you can maneuver the parachute, you can change course more often than that.  That makes you difficult to hit.

@Harper-ReinstateMonica AA isnt going to be firing at parachutists, but ground forces are, and stand a better chance of hitting you regardless of the aerobatics you want to do simply because of a high rate of fire and an easy to aim weapon - soldiers can take two approaches, either aim and take single shots, or put as much lead in the air as possible.  Either way, the parachute you have isnt going to allow you to dodge their shots, the altitude you drop at and rate of descent are…  I dropped with the British Parachute Regiment on a few occasions, it was fun.

@Harper-ReinstateMonica: That's what tracer bullets are for.

That only confirms that the bullets reaching you now are going where they expected you to be @JörgWMittag.  Again, you turned during the bullet's aim/fly time.

@Mast You see this in the opening of the movie Air Force One lol. Whether or not that's just HOllywood I have no idea. I think I heard someone say once though that the Special Forces sniping the guards while parachuting down was BS though.

Non-aerofoil chutes are more compact, cheaper, can be deployed at much lower altitude, keep the troops roughly together, and most importantly actively avoid collisions with each other (two simple drag chutes that get close to each other will get pushed apart by their own airflows).  Aerofoil chutes fly sideways at significant speeds, and not only do they not avoid each other but are actually pulled into an adjacent chute, both horizontally and vertically!

You’ve made some faulty assumptions there as the military does use square chutes for specialist parachuting, but rounds are popular for several reason:

Static-line round chutes are deployable en-masse from a much lower altitude / height than square chutes. The lowest that we can drop the round chute is 400’ (which is very low) versus circa 1500’ for a square.

They require the least amount of training to get the user competent to jump out of the two. Since the chute isn’t steerable, the soldier is reliant on the aircraft crew to make sure they are despatched in the correct position in space to arrive at the DZ. So their only job is to run forward when the light goes green and follow the guy in front out of the door then action their drills and brace.  Remember this is only their method of egressing the aircraft to get to the ground and fight and it’s not a particularly covert method of entry. This provides a large degree of force concentration, the largest number of people out of the aircraft and onto the ground in one go.

Square chutes are steerable and can be deployed in several ways and at much higher altitudes/ heights to allow the user to either free fall from height down to a low altitude before actuating they chute, or to use static line or pull immediately after jumping to fly in to the DZ from as far away as possible. This is a much more covert method of entry and requires a lot more training to achieve with inherently greater risks. Hence it’s not for everyone and isn’t usually given the media attention that round chutes get.  This boils down to some assumptions that people have that the government / military operations have almost infinite resource to train and keep people competent in high-end skills whereas almost the opposite is true and keeping it simple is usually the preferred option.

Also for emergency egress, the user may be unconscious from the ~18G ejection, so they wouldn't be able to steer anyway. And have exactly zero training.

Also, when you're hanging in mid-air, you make a rather nice target.  So you generally want to get on the ground fast, not hang around enjoying the view like a sport parachutist.

One factor may be that if the paratroopers deploy on "dumb" chutes, very rapidly, then they will tend to fall in the same staggered pattern all the way in.  If they all started wheeling around the place on steerable parachutes they would not just need to train in how to operate the chute but also in "flying" safely in a big pack.

@GrimmTheOpiner Yep! Moreover, if the launch happens with low visibility (e.g. night), being able to steer might be a liability since you don't even know in which direction to steer to avoid collisions.

Related to the last paragraph, reading about HAHO and HALO drops may be of interest

@JanHudec I had an uncle (now dead) who survived when his Lancaster was shot down by a German night-fighter. His position was one that was usually unsurvivable. He recovered consciousness with his chute open and no knowledge of leaving the aircraft or opening his parachute.

Its worth noting that some units like the SAS and US Marine Corps Recon do use ram air parachutes for case of inserting small numbers of high skill forces covertly. So its different parachutes for different roles.

@jamesqf: With ram-air parachutes, you can descend very rapidly (by diving the chute) before flaring out at low altitude.

Though, competent militaries generally don't shoot at enemy pilots who bailed out of a crashing bird, as they are hors de combat, out of the fight, and about to get captured. Totally different deal when 100 guys with tactical gear jump out of an intact C-130...

@Vikki But that would only matter for a HALO drop, and in that case you would both take a bit longer to get to the ground (more drag just because the chute is out instead of packed) and still present a larger target on the way down when compared to just using normal HALO procedure with a more traditional chute.

@AustinHemmelgarn you would use a square chute on a HALO drop and not a round as the soldier would want to steer to get to their drop zone after the free fall segment.

@user1937198 - Almost all special forces type units that insert via parachute use the ram. They also have longer, advanced courses rather than the basic 3 week course taught at... Benning? I believe that's still the basic parachute school in the US.

@Vikki: Only if you have been trained to do that, which takes time away from other training.

@Harper-ReinstateMonica, if the paratroopers are above their own territory, they are still a valid military target - if they survive, they will just be given new planes and get back to fight.

@RussellMcMahon I thank your uncle for his service, even if it's only symbolic. (From Berlin.)

@VolkerSiegel  Many thanks. That brought tears to my eyes. I'll pass that on to his son.
Post war my uncle continued dropping things out of aircraft - aerial aid drops in Africa. A Hercules at 50 feet dropping stuff must have been awesome. He said wheat in a sack often burst on impact. Wheat in two saks sometimes burst. Wheat in three sacks never burst. 3 sacks it was. I imagine youd want to avoid being hit by a triple bagged wheat bag from 50 feet.

There are serious military advantages to using a round "non steerable" parachute for mass tactical assaults.  (And there are sometimes military reasons to use ram air or steerable rounds, generally for special operations forces operating in small teams...)
First, keep in mind the purpose of "regular" airborne operations.  We aren't talking about inserting a highly trained and experienced 12 man Green Beret A Team behind enemy lines and land them all in a volleyball court sized clearing.
We are talking about dumping at least 500 fully equipped infantrymen on a drop zone that is probably at least two miles long and a half mile wide... we want them on the ground safely, with 100+ pounds of combat equipment, as quickly as possible, and we don't really care where on that DZ they land.  For larger operations, well have to use larger DZs or multiple DZs, and may well be dumping 20,000+ troops.  We also want the troops to be able to walk out of a relatively short course with a very low number of practice jumps, and be ready to make a combat jump for real the week they report to their unit. (There were two brand new E-1 privates who literally hadn't even reported to their units after Basic, AIT, and jumpnschool, whose first jump after jump school was a combat jump into Grenada with another unit - these privates were engineers and qualified to drive bulldozers, were so new they could be soared from their unit without costing serious combat capability, and thus available to jump in with the Rangers to drive away the heavy construction equipment the Cubans left parked all over runways the Rangers needed to sieze and put into service.  They jumped at 500 feet into combat as their "cherry blast", before they even met their own platoon leader.)
This thinking also applies to military emergency parachutes, like bail out and ejection seat rigs.  And those military pilots, by and large, are not parachutes, so it needs to work well the first time theybuse it, even if they are unconcsious or injured when the canopy deploys, it's too dark to see anything, and they don't know what's on the ground other than "bad guys".

Static line rounds are able to drop from FAR lower altitudes (even if you use a static line to open the ram-air automatically 20 feet after leaving the airplane, you still need it to open at least 3 times as high as you would a round.)

Non-steerable "round" parachutes can be deployed twice as fast as steerable rounds (to say nothing about ram air, which have to have even greater deployment separation than even steerable rounds), because you can use both doors in the aircraft simultaneously.  You cannot with steerable chutes, because to be steerable (whether round steerable or ram air), the parachute has to have forward drive.  Since an off-heading opening is common with combat equipment jumps (and hardly uncommon even with "Hollywood" jumps), the odds that two jumpers will open simultaneously on a closing course, separate by less that the width of the fuselage, is very high.  Even more so when you consider that line twists (which mean you cannot steer the parachute until they are cleared) are normal in static line jumps... my one collision and entanglement out of more than 150 military round canopy steerable jumps was caused by tight exit spacing and no time to react - we collided almost instantly on exit because we were flying at each other.  This is really not an issue with nonsteerable round parachutes - the jumpers tend to drift along the same rough angle of decent and always with the wind (and collisions and entanglements are much less of a safety hazard with nonsteerables - there is no chance of what is called a "death spiral" where the parachute are thrusting in opposite directions).

PVT Snuffy tends to steer toward what he is looking at.  This is common for all methods of moving.  And PVT Snuffy tends to focus on the jumpers closest to him, as he has been trained to try and pay attention the the jumpers around him for safety reasons.  So, PVT Snuffy tends to unconsciously steer towards the other jumpers, in addition to doing things like turning before looking, turning at low altitude (which is a leading cause of injury and death among skydivers), etc.  If you can't steer, you cannot steer into a collision.

Steerable canopies MUST be landed into the wind, and the jumper MUST get pointed into the wind at a reasonable altitude - 200-250 feet for a round steerable, and the landing pattern for a ram air starts at 1000 feet above the ground.  Most new jumpers have a hard time "reading" the wind and have to rely on being told what the surface wind direction will be (not always possible for a combat jump, and almost impossible for a night jump under operational conditions).  And you have to get the wind speed right with a steerable (round or ram air)- depending on the parachute, you're looking at adding 8-25 knots of ground speed to the wind speed if you land downwind. If the nonsteerable guy screws up wind direction?  Eh, not too big a deal - while you're supposed to "slip" into the wind direction, that's only a few knots of movement at best.  You are going to be landing downwind regardless, unless the winds are nearly non-existant.

Non-steerables are simpler and have less to go wrong (try being a jumper with an inverted canopy that has you flying backwards at 10 knots, or with a single jammed control line that means you are spiraling, or having both steering lines broken, entangled, or otherwise inoperable - now you are AUTOMATICALLY flying downwind, or you're a pilot who has ejected and is unconscious BECAUSE of the ejection), are easier (amd faster) to pack, and are cheaper to buy and maintain (which adds up when you're buying and maintaining equipment for an entire airborne division, 90% of whom will never need the capabilities of a steerable parachute.

If you're going to go into the trees, having a parachute that isn't producing any ground speed of its own is a GOOD thing - better to fall down through the trees and maybe get the canopy hung up, than to slam into a tree trunk at 10 or 20 knots like George of the Jungle.  And going into the trees is likelier for a combat paratrooper or pilot than it is for a civilian skydiver.

Lastly, for military bail out and ejection parachutes, it is entirely lilely you will be leaving the aircraft at several hundred miles an hour- a C-9 canopy will function properly at up to 300 knots, even if you are in a weird opening attitude and unstable.  That makes the C-9 canopy a damned good choice if you are planning on exiting an airplane because someone just shot you down and you don't have time to try and bleed speed.  Ram air?  Eh, not so much, generally.

As Arkem stated, they use both, but the primary reason for the use of a simplified parachute is just that it's simplified. In general Airborne troopers are supposed to fall where they are told to, as intended by the pilot, as instructed by the wing commander, as ordered by the REMFs.
When you give that same trooper the ability to adjust their flight path then you add in all manner of variables that are not only unnecessary, but dangerously unpredictable.


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What the preparations and procedures to consider for circumnavigation?
If one wants to circumnavigate the planet in a general aviation airplane, how can one do it and what are the important issues to consider?

Circumnavigating the planet Earth very popular hobby and a courageous endeavor. It is not an easy task and requires extensive preparation, planning and training.
What is circumnavigation?
Travelling around an island, a continent, or the entire world is called circumnavigation.
What is considered global circumnavigation?
Crossing all meridians is not considered global circumnavigation. If you  walk completely around either Pole, you have crossed all meridians, but this is not generally considered a circumnavigation.
A basic definition is a route which covers at least one great circle, and it passes through at least one pair of points of antipodal to each other.
What are requirements for circumnavigation by airplane?

The route should start and finish at the same point.
It should cross all meridians.
The course must be at least 22,858.729 miles (36,787.559 kilometers) long. This is length of Tropic of Cancer.
It must include set control points at latitudes outside the Arctic and Antarctic circles.

Do people do it?
Yes, a lot of people do it. Earth Rounders shares experiences of pilots who have flown around the world. It is categorized by engine types and solos etc.
What should I need to know?
A lot. Typically, people who flew around the world typically break their journey in 80-100 flights, or even more. It takes several months. Each flight of circumnavigation is significantly longer than a typical cross-country flight. It is much harder on many levels than an ordinary cross-country flight.
This website takes about the preparations thoroughly. It covers these section:

The Pilot
The Aircraft
Preparation
Survival equipment
The flight

Because of the longevity of this article, I am not duplicating it here as it contains all very useful information.
What are other people's experience?
There are many people who have thoroughly documented their journeys, the preparations, the hazards and difficulties they faced, and many pictures and videos. I am listing a few, which certainly are not the best ones, but give a decent idea about their trip.

Robert Gannon
Carol Ann Garratt
Flight Simulator X
Although this is in a simulation, but this trip is very thoroughly planned and it contains detailed path descriptions.

Would a flight from A up a meridian to the north pole, down the opposite meridian to the south pole and then back up the original meridian to A qualify? (Ignoring the ridiculous terrain covered!)

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What happens when all electrical systems in a helicopter fail?
I was thinking of asking this question at Worldbuilding, because it is for a book I am writing, but I hope this site will fit better. You can migrate this question if you find Worldbuilding more appropriate.
Let's imagine a flying helicopter, something like Eurocopter EC135. What happens if all electric equipment suddenly stops working? The reason of this failure is not important, just imagine all electricity is "gone" without any warning: all screens are black, all lights are gone, all communication is cut, GPS not working anymore etc.
I suppose the main engine is still running, no reason for sudden falling, right? But... is the pilot still able to fly it? Is it probable he would land successfully? Does the helicopter need any electricity to stay flyable?

Comments are not for extended discussion; this conversation has been moved to chat.

The reason for the failure is important actually. If all the electrical systems on an EC135 helicopter fail in your classic EMP scenario then the helicopter has to make an emergency landing as the jet engine is run by a computer system which requires electricity. A power failure of that system would most likely cause the engine to revert to manual backup, which means it's still running but you need to get it down ASAP. It's also possible the engine could fail entirely, in which case the helicopter would have to autorotate, i.e. land without engine power, which is survivable, if hazardous and extremely exciting.
If you are asking about a more straightforward electrical failure of flight instruments and comms with the engine still running then the pilot would continue to fly the chopper and use backup instruments to complete the flight. Even fully "glass cockpit" aircraft have a set of "steam gauge" instruments, some which require no power whatsoever, like an altimeter and airspeed gauge which use air pressure measurements, a magnetic compass for navigation, etc. Some will also have a backup Attitude Indicator which is powered by an isolated battery system and would give the pilot attitude reference even in cloud or poor visibility. So it's less of an event that you might imagine, although instrument failure is always an interesting experience, trust me.

It seems to me that you have a funny idea of what's exciting and interesting... :-)

Haha, fair enough @MichaelKjörling. I guess I was answering this with my novelist hat on rather than my pilot hat on. If you want to put a character in peril an engine failure is must more visceral for the reader than an instrument failure.

They had an instrumentation problem on Apollo 13. Made for a reasonable amount of drama. :-)

Yeah, but first there was an explosion! And it was in space! Great example though @MichaelKjörling

Yeah, but at that point, they didn't know they'd had an explosion. All they really knew was that a bunch of instruments were showing readings that they weren't supposed to show at that point in the flight and that systems were dying left right and center... So it was still an instrumentation problem at that point. A pretty dramatic one.

Citation needed on the loss of engine power. FADEC engines are typically designed so that the HMU remains in its last position during a FADEC failure, so the engines will remain running. (A plausible scenario being the FADEC is resetting and will come back up in a few seconds) Another example is the QF32 accident, loss of control wiring resulted in an engine stuck running after landing.

@user71659 HMU appears to refer to the final valve that actually controls fuel flow into the engine? My understanding is that gas turbines are fairly unstable without active (hydraulic or electronic) controls, and an engine with a fixed fuel valve would quickly over-speed or stall. As for QF32, my understanding is it's probably the throttle signals from the cockpit to the FADEC, not the FADEC itself.

@SomeoneSomewhere The hydromechanical unit consists of electromechanical fuel valves as well as hydraulic servocontrols (variable stator vanes and such). Stability is more difficult in large turbofans instead of turboshafts, and during transient operation. The engine here has only one stage of anything, and uses a centrifugal compressor. It should be fairly robust, especially since you're just trying to get to the ground.

In fact, I got a hold of the EC135 flight manual and there is a procedure for total FADEC failure/stuck fuel metering valve. It involves setting a target torque with the twist grips (mechanical fuel control) and then LAND ASAP. Thus, there is no need for operational FADEC for continued engine operation.

Thanks for the research @user71659, I've edited accordingly.

@user71659 Do you happen to have a link handy?

@MichaelKjörling Not an official link, but type the obvious words into you favorite search engine.

It turns out you picked a particularly susceptible helicopter. Unlike other helicopters which are still largely mechanical, the EC135 has digital engine control. The simplest solution is to pick a different helicopter.

I suppose the main engine is still running, no reason for sudden falling, right?

I have some bad news. The EC135 uses a sophisticated computer to control the engine. Safety is typically guaranteed with multiple fully redundant computers and wiring paths, but if all electrical devices fail at the same time the turbine is no longer under control. To quote Wikipedia:

If a total FADEC (Full Authority Digital Engine Control) failure occurs, the engine fails.
Upon total FADEC failure, pilots have no manual controls for engine restart, throttle, or other functions.

No electricity, no engine.
Fortunately we can safely assume the turbine is at least engineered to fail safe, it won't spin itself out of control. Unfortunately that means "shut down". So yes, you lose main engine power.
Fortunately there will be some residual power in the turbine as it spins down. That plus the energy in the rotors can be used to autorotate as covered by GdD.

But... is the pilot still able to fly it? Is it probable he would land successfully? Does the helicopter need any electricity to stay flyable?

Yes, fly-by-wire helicopters are very rare. While they will no longer be augmented, stabilized, or power assisted, the mechanical linkages will still work. And they'll be able to autorotate, as @GdD described, and as demonstrated in this Smarter Every Day video.
However, sudden loss of engine power, and of all their flight control augmentation, and any electronic instruments, might confuse the pilot. An experienced pilot will still autorotate and land, but it probably won't be as pretty as in the video. I found an actual crash report of an actual EC-135. The experienced pilot thought they'd lost all engine power, went into autorotation, and landed with no injuries, but trashed the aircraft. In fact the engines were fine, but they lost their Stability Augmentation System. Reading that report might help with your book.
Helicopter pilots train for a sudden loss of power and emergency landing. If they still have control over the aircraft they will be able to land it.

The EC135 is not a FBW helicopter. It has conventional mechanical linkages with hydraulic assist (like power steering or a Boeing 737). It has electronic augmentation, but it isn't necessary for basic flight. The first FBW civilian helicopter, the Bell 525 came 10 years after the EC135.

@user71659 Did I misread the Defense Aerospace article? Was that an experiment?

@Schwern The article clearly states is was a demonstrator... a production model that was retro-fitted with the Fly-By-Light system.

@Schwern The article describes an event from 2002 while EC135 are flying since 1996. But indeed it's written in a way that's confusing  without researching further. Although it mentions that it's NH90 that will be the first production fully FBW helicopter.

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How to define pitch angle for a passive variable pitch propeller?
This question is a continuation of the question How to calculate variable pitch propeller parameters? mentioned before.
Actually, the theory described as an answer on the question above is the exact one i am using at the moment.
However at the current stage, i want to make the calculations only for a 2D sectional symmetric airfoil, supposing a rectangular-shaped blade and not considering any twist.
Also i forgot to mention before that my idea is to attach a rod on the root of the blade, set in an offset of the aerodynamic centre Xa.c (variable) and place a gear on it to connect with the opposite blade and synchronizing it. Also counterweights will be attached on the rod for balancing purposes but this is for later studies.

In this case i have ended up with the following formula for the pitching moments:
dM = dMa.c - dL*Xa.c
As can be seen here dM and dL are correlated there are two unknown there which are the lift coefficient Cl and angle of attack α.
from literature i found that Cl can be expressed as Cl0 + Cla* α and then to 2π*α (from thin airfoil theory), as well as Cm = 0 for symmetric airfoils.
Using these assumptions and moment equilibrium  i end up on α=0 which doesn't make sense.

If your axis of rotation is a hinge, it does make sense.

But let's say this is for the take off condition where V=0 m/s and RPM = 2000. What about cruising and landing conditions?? Will α still equal to zero?

At $\alpha$ = zero there is no thrust. You'll need to compute required thrust to overcome aeroplane drag, then compute $\alpha$ required from speed at radius r and $C_L$. If there is no twist, $\alpha$ will be a different value at every r.

I suppose that when there is no twist, α will be the same along the blade (that's the reason i am using simple rectangular shape for the blade). Also, Thrust is calculated from dT=dLcos(φ)-dDsin(φ) where dD = Cd1/2 ρV^2 cdr, where dD the drag of the propeller blade and Cd the drag coefficient (unknown). if i use the α=0 (no thrust, dL =0) the dT = -dDsin(φ) =0 which implies drag =0. I understand that at static thrust condition these parameters will be zero. The question is how to calculate them for the next flight operation

Thrust is computed from L. Torque to drive the blade is computed from D. Unfortunately, D can never be zero, there is a $C_{D0}$. Only a blade with twist has an $\alpha$ that is continuous along the blade, for airspeeds other than zero. If $\alpha$ = 0 there is no propeller thrust.

So the equations i ve shown above are incorrect? Because Drag is given from the thrust there which is zero. However, the problem is how to find L which implies Cl which implies α for cruising condition. If i can find this parameter then i can find everything i assume.

You're using definitions for wings. The term lift is for wings, not propellers, they deliver thrust. Drag is for wings, torque for propellers. Most of the aerodynamic data for the profiles can be used, but 1. the propeller rotates which makes the aerodynamics a bit more complicated than those of a wing 2. $C_L$ should be called $C_T$ for thrust 3. It works the other way around: find $\alpha$ from airspeed and local rotational speed, find $C_T$ ($C_L$) for this $\alpha$ from section data, find thrust from integrating over the propeller length.

Ok let's start from 3. I know airspeed and i know rotational speed. From them i am calculating the resultant velocity Vr. Now i can find the flow angle φ (sinφ = Vinf/Vr). Now, how α can be computed only from airspeed and rotational speed? Also Ct is interdependent with Thrust, how can be calculated if both are unknown.?


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What calculations are needed for counter-weight balancing on a propeller?
I am designing a small-scale propeller and i have managed to design on optimized geometry for the blade. However the blade pitch angle adjusts according to different flow speeds so i need to mount counterweight to maintain the balance of the blades. What i need to consider for the calculations?
Let's say i know already the counterweight shape, mass, and mounting place.

40 cm overall diameter, 15 cm blade length, max rpm = 7000

it is about a project i am carrying out

Actually the propeller is going to be tested alone in a wind tunnel. Not any flight test would be conducted. However i want to investigate its feasibility on small UAVs (airplanes). So i have already designed the whole propeller system and the only thing remains is to balance the blades spinning of the blades while the propeller will be subjected to different flow velocities


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What Inertial effects should be considered to achieve blade stability on a Variable Pitch Propeller tailoredby aerodynamic loads?
I am developing a self adjusting variable pitch propeller tailored by its pitching moments due to aerodynamic loads setting the blade rotation axis in an offset from the aerodynamic center as shown below.

At the moment i am trying to attain static and dynamic stability of the blade due to mass/Inertial effects. I have created a sketch as shown on the following figure, based on my so far understanding on inertial effects which represents a blade airfoil section and the centrifugal force (OR NOT??) acting on a mass particle.

Mass: ρdxdy*dr
Offset from Pitch Axis x
Offset from longitudinal Axis y
Force: (ρdxdy*dr)*rω^2
Component along the x-axis: (x/r)*Force
Pitch Moment: y*(x/r)*Force
And now comes the difficult part. I suppose that here i need to consider the theory of three dimensional kinetics of a rigid body and angular momentum, however i am confused on defining the coordinate systems on the blade as well as which exact moments and products of Inertia have to take into account. If i am not wrong, here a fixed coordinate reference system and one that rotates with the twisting of the blade have to be defined.  The question is where the coordinate reference system has to be defined; on the blade axis of rotation, C.G, or somewhere else?
Finally i have informed that  to compute the blade's dynamic stability, i will need also to consider blade flapping (i.e., motion of the blade out of plane with the rotation of the propeller), and pitching (or twisting) and this is another part i am struggling too!

@mins Actually i have managed to compute twist and chord distributions but i suppose that this is not an issue for the current situation (It is just a sum up on the overall pitch angle (twist) and a variation on the coordinates(chord length)). This is a propeller for small UAVs but i assume theory is similar with heli's tail rotor yes!


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In ARINC 429 where are the bits indicating the hex value of Equipment ID?
I'm a newbie in ARINC-429 standard; the following might be a very simple issue for you.
In ARINC 429 the LABEL field  is covered by the bits 1 through 8; which bits are covering the Equipment ID?
For istance: Equipment ID 002 hex (Flight Management Computer) would require 12 bits to be represented. Where are located these bits in the ARINC word, so that the receiver could read the it?
Thanks for your assistance.

ARINC 429 is a unidirectional one-to-many (broadcast) transfer bus with a maximum of 20 receivers.

SourceDesign with four unidirectional ARINC 429 buses
There is normally little need to identify the source, as there is only one. So there is no field in the protocol word dedicated for ID transmission. As visible in the previous image, bidirectional exchanges require two buses and several ports to connect them. The receiving unit (LRU) can determine from the port used which source feeds the bus.
However label 377 (octal) can be used to transmit, from time to time, the equipment ID if desired. Example: ID 10D (hex):

Source: ARINC specifications
Note: On the other hand, SDI bits can be used by the source to select the receiver.

SDI - Source/Destination Identifier can be used to identify either the source or destination of multiple LRUs of the same type. Tuning controls will use it to select the destination LRU. e.g.; VOR is controlled with Label 034. SDI will indicate whether the tuning label is intended for the left or right VOR. With 3 IRS on board, they each use SDI to indicate  which IRS the data originated from. This allows receiving LRUs to select a specific source IRS (Left, Center, Right) for their data. When LRUs are installed, discrete hardwired inputs set the LRU's position.

Your answer was a big clarification of my doubt. Thank you so much, I really appreciate it.

As said earlier label 377 can be sent for Eqip ID. Alternatively, if you are the source then you can reserve few data bits for equip id and have it documented in your ICD.

The equipment ID should only ever go in the label 377, anything else isn't going to be compatible with the receiving equipment.


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Why are flights from Fuzhou or Xiamen to Taiwan not allowed to overfly Taiwan strait?
The flights from Kinmen or Matsu to Taiwan always overfly Taiwan strait. Why can't the flights from Xiamen or Fuzhou use the same route, fly over Kinmen or Matsu first and then to Taiwan directly, but need to fly away from the strait?

Because Kinmen and Matsu are governed by Taiwan, ROC, while Xiamen and Fuzhou are governed by Fujian, PRC. PRC and ROC have some complicated political issues covering almost all civilian and military topics (finances, trading, travelling, etc), and air traffic is one of them. Less than a decades ago IIRC situation was even worse, e.g. those flights need to actually land (actually physically touch the ground) in HK before entering the other side.

You may expect such curved flights to end some time in the near future, but ROC and PRC fought a pretty big civil war in the 1940s and continue to fight for the UN position till 1971. Kinmen in particular, was still receiving artillery attacks until 1978. Give it some time.

The answer to this question is covered in good detail in this news article in The Straits Times. Essentially, it's a political dispute between the countries of China and Taiwan.